I agree with Leathermang's assessment. On newer vehicles, the car itself will rot into the ground long before added wear on the bearings has a chance to do any harm. Thus, lighter oils for better fuel economy make a little more sense.

I also sometimes think those numbers are a bit contrived anyway, as there is more to the lubricating properties of a fluid than just its viscosity.

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Keep everything as simple as possible-but no simpler--Albert Einstein